The keys 1, 2, 3 and 4 are called Zoom in the menu.
But they also changes the size of the window.
I think they should only zoom, not change the window size.
Like if you zoom a image in a image manipulation program like GIMP you will zoom the image in and out, but the window size does not change, there will be scroll bars to use if not the entire image can bee showed.
http://avidemux.org/smif/index.php/topic,16955.0.html (http://avidemux.org/smif/index.php/topic,16955.0.html) might be of some interest. I agree that Avidemux should not resize its window by default but zoom the video smoothly to fit into the available space. This would probably require massive changes in the application though. Patches welcome :)